1. Field of the Invention
The present invention relates to a zoom lens system for lens shutter cameras.
2. Description of the Prior Art
In the recent years, attempts have been made to design compacter zoom lens systems having lighter weights under the trend to manufacture compacter cameras. Especially, compact zoom lens systems having zooming ratios on the order of 2 are demanded also for lens shutter cameras which do not permit exchange of lenses.
As a zoom lens system satisfying this demand, there is known a lens system which comprises a front lens group having positive refractive power and a rear lens group having negative refractive power, and is so adapted as to perform zooming by varying the airspace reserved between the two lens groups.
Such a zoom lens system can be made compacter by minimizing radius of curvature of each lens component so as to increase power of each lens surface. However, such a measure will increase variations of aberrations during the zooming operation, cannot correct aberrations favorably and will not allow to obtain a sufficiently large zooming ratio. Further, in order to minimize variations of aberrations during the zooming operation, correct aberrations favorably and increase zooming ratio, it is necessary to increase the number of lens components or enlarge the zoom lens system as a whole contrary to the demand of compactness.
In order to correct this defect, Japanese Unexamnined Published Patents Application Nos. 148414/61, 259216/61, 295524/61, etc. disclose zoom lens systems using graded refractive index (GRIN) lenses. However, all these conventional zoom lens systems have short distances between the final surface of the lens system to the image plane, i.e., short bback focal lengths undesirable for practical use. In addition, these conventional zoom lens systems have relatively high ratios between the total lengths of the lens systems and focal lengths at the wide position, i.e., so-called telephoto ratios, and are not compact lens systems.